Aluminum stands out as a favored choice in windows, doors and other fenestration products because of its structural longevity and its high resistance against corrosion, deflection and wind load. This outstanding material is lightweight and is quick and simple to extrude, machine and fabricate into virtually any form. Optimizing thermal performance contributes to energy efficiency and helps reduce associated heating and cooling costs. The one disadvantage to the use of aluminum in windows and doors is that it is a highly conductive material. Conduction is heat transfer which can be controlled by the addition of low-conductance thermal barrier materials. When a thermal barrier process is completed, there is no aluminum contact from the exterior to interior. Thus, transfer of heat is interrupted, resulting in an energy-efficient, insulating thermal barrier.
Current technology to achieve a thermal barrier uses pre-formed polyamide struts which are slid into grooves on opposing walls of the tube and then are crimped simultaneously into both halves of the aluminum making a tubular shape. The weakness with this method is that crimping of the polyamide struts cause bowing and distortion of the aluminum halves resulting in a finished part that is not within tolerance. In addition, the shear force developed between the polyamide struts and the aluminum is not sufficient to provide composite bending or adequate torsional strength of the tube. In an attempt to resolve these issues, mandrels are inserted into the tubular shape before crimping. The mandrels provide support for the aluminum and minimize distortion due to the crimping process. The mandrels also allow for a tighter crimp on the polyamide thus increasing the shear force. As an alternative, interior webs are extruded as part of the aluminum section to provide support similar to the mandrels. Both these solutions require the added expense of either using mandrels during the crimping process or adding more metal within the profile to help support the crimping while adding minimal additional shear or torsional strength.
Another option to using polyamide struts as the means to provide a thermal barrier is to use a cast-in-place polyurethane. Typically cast-in-place thermal barriers are poured into a cavity in a single extrusion. Following curing of the polyurethane, the back side of the cavity is then removed creating a structural thermal barrier joining the two halves of the extrusion. For a tubular extrusion, with two cavities on opposing sides of a tubular shape, it is not possible to remove a metal bridge connecting the two halves of the extrusion.